CHAPTER III 



THE NEURON 



As we have seen in the last chapter, the functions of irrita- 

 bility, conduction, and correlation are the most distinctive fea- 

 tures of the nervous system. Like the rest of the body, the 

 nervous tissues are composed of cells, the irritability of whose 

 protoplasm is of diverse sorts in adaptation to different func- 

 tional requirements. Each sense organ, for instance, is irri- 

 table to its own adequate stimulus only (see pp. 25, 69). The 

 functions of correlation and integration of bodily actions cannot 

 be carried on by the nerve-cells as individuals, but they are 

 effected by various types of connections between the different 

 cells in the nerve-centers. The character of any particular cor- 

 relation, in other words, is a function of the pattern in accord- 

 ance with which the nerve-cells concerned are connected with 

 each other and with the end-organs of the reflex arcs involved. 

 The conducting function of nerve-cells is, perhaps, their most 

 striking peculiarity, and their very special forms are due largely 

 to the fact that their business is to connect remote parts of the 

 body so that these parts can cooperate in complicated move- 

 ments. 



Not all of the cells which compose the central nervous system are nerve- 

 cells. The brain and spinal cord are surrounded by three connective-tissue 

 membranes (dura mater, arachnoid, and pia mater, in the aggregate 

 termed meninges) whose functions are chiefly protective and nutritive; 

 from the inner membrane, the pia mater, blood-vessels, and strands of 

 connective tissue extend into the true nervous substance. In addition to 

 these non-nervous elements which grow into the central nervous system 

 from without, the substance of the brain and spinal cord contains a sup- 

 porting framework composed of ependyma and neuroglia or glia cells which 

 develop from the primitive embryonic nervous system (the neural tube, see 

 pp. 106, 116), but are not known to perform nervous functions, though 

 nutritive and other functions have been ascribed to them (see p. 104). 



The true nerve-cells are called neurom. There has been a 

 long controversy regarding the way in which the neurons of the 



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